1. Field of the Invention
The present invention relates generally to devices, systems and methods for ultrafast optical applications, and more specifically to a nonlinear optical mirror, and uses of same in optical system, providing stronger modulations of the reflectance or transmittance in the visible and near-infrared spectral ranges at lower pump energies than conventional nonlinear optical materials or devices.
2. Description of Related Art
Materials with a strong power-dependent reflectance, absorptance or transmittance are keys for the development of all-optical applications such as optical signal processing, pulse compression, passive protective devices and medical image processing. A trade-off exists between the strength and the response time of a nonlinear process and the transparency of a material. The nonlinear optical response of a material is strong when the optical field resonates with the frequency of an allowed electronic transition, but this also leads long-lived excitations that are much slower than electronic processes that induce a nonlinear polarization. As a general rule, the strength of the nonlinear polarization terms induced in a material increases as the optical bandgap of the material decreases. As a consequence, transparent materials with an ultrafast nonlinear optical response have weaker nonlinearities in the visible spectral range compared with transparent materials in the infrared spectral range. For these reasons, nonlinear optical devices are more easily implemented in the near-infrared and infrared spectral ranges. However, in all cases, since the nonlinear response of common materials is typically weak, when incorporated in device configurations they require large switching energies and/or long interaction lengths to produce significant effects. In addition to these restrictions, other restrictions such as fulfillment of wave-vector matching conditions further complicate the incorporation of these materials into active optical devices.
Known examples that are particularly relevant to the present invention include: nonlinear mirrors and ultrafast optical shutters. Nonlinear mirrors have been realized by: the combination of a second harmonic generation (SHG) crystal and a dichroic mirror and used to demonstrate intra-cavity passive mode-locking operation for picosecond-pulse generation in the visible range; Bragg-periodic structures comprising semiconductor layers with Kerr-type optical nonlinearity in the infrared spectral range have been reported. Ultrafast optical shutters or loop mirrors are known to be realized using a nonlinear Sagnac interferometer wherein an intra-loop nonlinear optical element is placed off-center to define the opening time of the shutter. The nonlinear optical element in these cases is known to be a Kerr medium, a saturable absorber or a high gain laser media.
The rapid-growth development of biomedical applications and micromachining using ultrafast optical pulses in the visible spectral range creates a strong need for developing ultrafast optical devices that operate at visible and near-infrared wavelengths. These devices are expected to enable the all-optical control over the spatial and temporal irradiance profiles of high energy optical pulses and could find multiple applications in a variety of optical technologies.
Noble metals are known to have an extremely large and ultrafast NLO response in the visible spectral range, much larger than most known organic or inorganic materials, but are seldom used as NLO materials due to their limited optical transparency and large reflectance in the visible (Vis) and near infrared (NIR) spectral ranges. Ultra-thin layers of noble metals can be semi-transparent in the visible range if their thickness is around the skin depth of metals (typically between 10 to 20 nm). Thicker metal layers rapidly loose transparency and become highly reflective due to the inherent large admittance contrast between metals and the dielectric environment. It is well known in the art that the dielectric environment of a metal can be engineered, by nanostructured dielectric layers, to relief the admittance contrast between the metal and its environment. Using this approach, it is possible for thin layers of noble metals, with a thickness of several times its skin depth, to be highly transparent within the spectral region where the optical response of the metal arises from their intraband electronic transitions. The large admittance mismatch, or alternatively permittivity contrast, between metals and dielectrics makes it possible to engineer compact nanostructures with fewer layers and unique optical properties compared to all-dielectric nanostructures. Induced-transmission filters (ITFs) and metal-dielectric bandgap structures (MDPBGs) are known examples of such nanostructures. The increased transmittance in such nanostructures arises from a large decrease in reflectance and an increased absorptance within the metal layers. For these effects to be significant, the excitation of surface plasmon polariton modes must be avoided since they lead to parasitic absorptance and decreased transmittance or reflectance. Hence, metallic nanostructures offer a unique opportunity to engineer optical devices with tailored linear optical properties in the Vis and NIR spectral ranges.
The ability to engineer the linear absorption in a thin metal layer within a nanostructure is also important because the NLO response of noble metals arises from the electron and lattice heating caused by the absorption of energy from an ultrafast optical pulse. The linear and NLO responses of noble metals are determined by the inherent electronic properties that define their dielectric permittivity. The electronic properties of noble metals in the visible spectrum are characterized by two separate mechanisms, namely interband and intraband transitions, that dominate in different spectral regions within the visible spectrum, and consequently lead to different properties. Electronic interband transitions dominate in the visible or ultraviolet (UV) spectral region and arise from bound electrons excited from fully occupied electronic states within the d-band, below the Fermi energy level, to the half-filled s-p electronic bands in the conduction band. At lower energies, electronic intraband transitions occur from free electrons stimulated within the conduction band. When a metal film is excited with an ultrafast optical pulse, the absorbed optical intensity raises the temperature of the electron cloud and smears the electronic distribution around the Fermi energy (Fermi-smearing), causing a very strong change of the dielectric permittivity of the metal around the interband transition onset. The latter is defined by the wavelength or photon energy where the electronic properties stop being dominated by interband transitions and start to be assigned to intraband transitions.
Known examples of nonlinear optical devices exploiting the nonlinear optical response of thin noble-metal films are induced-transmission filters (ITFs) and metallo-dielectric bandgap structures (MDPBGs). These devices are known to amplify the response of a single thin metal film and typically display nonlinear reflectance or transmittance changes smaller than 10% at moderate fluences smaller than 10 J/cm2.
The unique linear and NLO properties of metallic nanostructures open the opportunity to develop a wide range of all-optical applications in a spectral region where materials with ultrafast and strong NLO response are very scarce and where there is a need to develop a technology that could allow a new generation of all-optical devices for biomedical, machining, and laser applications. It is the intention of the present invention to provide for such an industrial need.